In the vehicle onboard reception, as well as in the indoor reception, of digital television (DTV) broadcast waves, multipath propagation generated by diffraction, scattering, and reflection of surrounding objects such as buildings gives rise to locally significant decline in signal intensity (fading), and entails significant waveform distortion, in the received signals, such that stabilized receiving of signals is problematic. For reception in such an environment, diversity reception using a plurality of receiving antennas is effective and thus put into practical use. A generally performed technique for diversity combining is maximal ratio combining, in which combining is carried out by matching the phases of the respective signals received at each antenna and at the same time weighting the received signals by carrier-to-noise ratio (CNR). Maximal ratio combining positively improves the post-combined CNR of the signals. In particular, with DTV schemes that adopt orthogonal frequency-division multiplexing (OFDM), carrying out maximal ratio combining on a per-subcarrier basis yields frequency diversity effects, wherein considerably ameliorative effects are obtained. The reception quality is improved, and the extent of the area over which reception is possible is improved, in accordance with the number of diversity branches.
Meanwhile, an alternative for improving the reception quality and extending the signal-receivable area is the idea to use an antenna having directivity. The use of directional antennas improves the multipath reception state by limiting the directions along which received signals arrive, and thus is promising for reducing fading. In addition, Doppler spread during high-speed movement is suppressed, wherein with OFDM schemes, which tend to suffer from inter-carrier interference, combining diversity reception and directional antennas is advantageous. However, unlike the situation with stationary reception, the directions in which the received waves arrive are diverse, such that in general it is necessary to prepare numerous antennas having directivity in differing directions, which requires switching and selecting among the numerous antennas to employ them, or requires conjunction with diversity reception such as described above and combining signals to receive them.
FIG. 9 is a block diagram showing the configuration of a diversity reception device of a first conventional example. In the diversity reception device of the first conventional example shown in FIG. 9, in vehicle onboard reception of DTV broadcast waves, a method for improving the quality of reception by using a plurality of antennas having directivity and using switching selection and combining in conjunction is proposed (e.g., see Patent Literature 1).
In FIG. 9, the diversity reception device of the first conventional example comprises front-beam antennas 5101 and 5201, rear-beam antennas 5102 and 5202, an antenna switch 5111, tuner units 5171 and 5271, gain control units 5135 and 5235, OFDM demodulation units 5145 and 5245, level detection units 5142 and 5242, a diversity combining unit 5303, an error correction unit 5304, and an antenna control unit 5301.
The antenna switch 5111 selects either the front-beam antenna 5101 having directional characteristics in the forward direction of a vehicle or the rear-beam antenna 5102 having directional characteristics in the backward direction of the vehicle, and selects either the front-beam antenna 5201 or the rear-beam antenna 5202. Received signals from the selected beam antennas are demodulated by the OFDM demodulation units 5145 and 5245 via the tuner units 5171 and 5271, respectively, then combined by the diversity combining unit 5303, subjected to error correction by the error correction unit 5304, and outputted as a transport stream (TS).
In the OFDM demodulation units 5145 and 5245, the respective outputs of the level detection units 5142 and 5242 are used: to control the gain of the tuner units 5171 and 5271 via the gain control units 5135 and 5235 to keep appropriate input levels of the OFDM demodulation units 5145 and 5245; and as received power information for determination information for antenna switching selection at the antenna control unit 5301. In addition, error rate information at the error correction unit 5304 is also used for the determination information for the antenna switching selection at the antenna control unit 5301.
FIG. 10 is a flowchart showing a procedure of the antenna switching selection at the antenna control unit 5301 in the diversity reception device of the first conventional example. In selecting the antenna having directional characteristics in the forward direction or the backward direction, the tuner unit 5171 or 5271, the OFDM demodulation unit 5145 or 5245, or the like is shared in the diversity reception device of the first conventional example. In addition, also due to restrictions of the response time of the gain control unit 5235 and the like, it is necessary to detect an average reception level over a time of about several hundred milliseconds. Thus, in order to check the reception state in each direction, the antennas cannot be switched indiscriminately.
Thus, first, the antenna control unit 5301 observes a reception error rate. When determining that the reception error rate exceeds a criterion and the reception state has worsened (i.e., when a switching start condition is met), the antenna control unit 5301 enters a sequence of the antenna switching selection (step S6001). Specifically, when the switching start condition is met at step S6001, the antenna control unit 5301 detects the average reception level of the front-beam antenna 5101 or 5201 over a period of several hundred milliseconds (step S6002). Then, the antenna control unit 5301 switches the antenna switch 5111 and detects the average reception level of the rear-beam antenna 5102 or 5202 over a period of several hundred milliseconds (step S6003). Then, the antenna control unit 5301 compares both average reception levels (step S6004), switches to the antenna having a higher level and keeps this switching state for several seconds to several ten seconds (step S6005), and returns to the determination of the switching start condition (step S6001).
According to this configuration and procedure, in the diversity reception device of the first conventional example, it is proposed to use the switching selection and signal combining in conjunction, whereby while increase in the chain number of cost-elevating later-stage tuner units and OFDM demodulation units is held down, the antennas are lent directivity, and the number of antennas is increased, which serves to improve the performance.
For example, in the diversity reception device of the first conventional example, a control procedure with improved antenna switching selection at the antenna control unit 5301 is proposed. FIG. 11 is a flowchart showing the control procedure with improved antenna switching selection at the antenna control unit 5301 in the diversity reception device of the first conventional example.
Similarly to the procedure in FIG. 10, since, in order to check the reception state in each direction, the antennas cannot be switched indiscriminately, the antenna control unit 5301 initially observes a reception error rate, and when determining that the reception error rate exceeds a criterion and the reception state has worsened (i.e., when a switching start condition is met), the antenna control unit 5301 enters a sequence of the antenna switching selection (step S7001). Specifically, when the switching start condition is met at step S7001, the antenna control unit 5301 detects the average reception level of the front-beam antenna 5101 (or 5201) over a period of several hundred milliseconds (step S7002). Then, the antenna control unit 5301 switches the antenna switch 5111 and detects the average reception level of the rear-beam antenna 5102 (or 5202) over a period of several hundred milliseconds (step S7003). Then, the antenna control unit 5301 compares both average reception levels, switches to the antenna having a higher average reception level among the front-beam antenna 5101 (or 5201) and the rear-beam antenna 5102 (or 5202) (step S7004), and waits for a given period of time (step S7005).
The control procedure shown in FIG. 11 differs from the procedure in FIG. 10 in that, with regard to antenna switching selection for another chain, switching selection is performed without actual measurement of an average reception level. In other words, at step S7004, when the front-beam antenna 5101 (or 5201) is selected, the antenna control unit 5301 performs switching selection of the front-beam antenna 5201 (or 5101) in the other chain as well, and when the rear-beam antenna 5102 (or 5202) is selected, the antenna control unit 5301 performs switching selection of the rear-beam antenna 5202 (or 5102) in the other chain as well (step S7006). Then, the antenna control unit 5301 waits for a given period of time (step S7007) and returns to the determination of the switching start condition (step S7001).
In addition, in the control procedure shown in FIG. 11, the idea is disclosed that the average reception level detection in the other chain is omitted to reduce deterioration of reception during switching and average reception level measurement. FIG. 12 is a block diagram showing the configuration of a diversity reception device of a second conventional example. In the diversity reception device of the second conventional example shown in FIG. 12, a method for improving the quality of reception by using a plurality of antennas having directivity and using switching selection and combining in conjunction for vehicle onboard reception of DTV broadcast waves is proposed similarly to the first conventional example. Furthermore, in the second conventional example, the idea to previously combine signals from a plurality of antennas to make a single diversity branch is also disclosed in addition to simple antenna switching. In addition, the idea to use, as a switching start condition, a reception level or a CNR or use a change of a travelling direction or a speed with a steering angle sensor or a gyro sensor is also disclosed (e.g., see Patent Literature 2).
In FIG. 12, the diversity reception device of the second conventional example comprises front-beam antennas 8101 and 8201, rear-beam antennas 8102 and 8202, an antenna switch 8111, tuner units 8171 and 8271, OFDM demodulation units 8145 and 8245, a diversity combining unit 8303, an error correction unit 8304, a TS decode circuit 8311, an antenna control unit 8301, a steering angle sensor 8312, and a gyro sensor 8313.
The antenna switch 8111 selects either the front-beam antenna 8101 having directional characteristics in the forward direction of a vehicle or the rear-beam antenna 8102 having directional characteristics in the backward direction of the vehicle, and selects either the front-beam antenna 8201 or the rear-beam antenna 8202. The difference from the first conventional example is that the front-beam antenna 8101 and the rear-beam antenna 8102 or the front-beam antenna 8201 and the rear-beam antenna 8202 can be combined to perform output. Then, received signals at the respective chains are demodulated by the OFDM demodulation units 8145 and 8245 via the tuner units 8171 and 8271, respectively, combined by the diversity combining unit 8303, and subjected to error correction by the error correction unit 8304, and information of images and sounds are taken out therefrom by the TS decode circuit 8311. For determination information for antenna switching selection, the antenna control unit 8301 uses not only received power information and error information similarly to the first conventional example, but also a CNR and a channel estimation result.
FIG. 13 is a flowchart showing a control procedure of the antenna switching selection at the antenna control unit 8301 in the diversity reception device of the second conventional example. Similarly to the first conventional example, in switching selection of directional antennas, in order to check the reception state in each direction, the antennas cannot be switched indiscriminately. Thus, in the conventional example as well, after it is determined that the reception state has worsened, a sequence of the antenna switching selection is entered (step S9001). In other words, the idea to set a switching start condition is in common. However, the idea is also disclosed that, as a switching start condition, a determination is performed when the received power or CNR at each of all diversity branches is less than a determined threshold, or a change of a travelling direction and a speed are used with the steering angle sensor and the gyro sensor (step S9002).
With regard to antenna switching selection after the switching start condition is met, when the received power is less than a threshold at step S9002, the front-beam antenna 8101 and the rear-beam antenna 8102 or the front-beam antenna 8201 and the rear-beam antenna 8202 are combined to perform output and are used as a diversity branch (step S9003). The idea is also disclosed that, similarly to the first conventional example, when the received power is not less than the threshold at step S9002, the antenna switch 8111 is switched, a CNR or a transmission path estimated value is detected, an antenna having a higher CNR or a less variation of the channel estimation determined as an antenna having a better quality of reception, and switching selection is performed (step S9004).
Furthermore, reception level is used, but with reception level, unlike CNR, accurate determination cannot be performed in the case of occurrence of variation in the gain of a tuner unit or the like, in a multipath, or in a Doppler environment. Thus, the idea is also disclosed that the relationship between a reception level and a CNR is previously measured and evaluated and is retained in the form of a table and conversion is performed with the table.